1. Field of the Invention
The present invention relates to systems and methods for signal processing, and in particular for delaying a signal via a continuously variable optical delay line.
2. Discussion of the Related Art
The technical pursuit to provide transmission media with greater bandwidth and higher data rates to efficiently and reliably convey signals (e.g., video and/or audio) has lead to increased research and development in the fiber optics domain and the deployment of fiber optic channels, interfaces and associated devices. Since the invention of the telegraph, there has been a constant push to provide data at higher and higher rates. For example, RS-232 once was the standard employed to attach terminals. Then, technologies such as 10 Mbps Ethernet and 4/16 Mbps Token Ring were developed and replaced RS-232 as the standard. The next generation of transmission technologies included Fast Ethernet (100 Mbps) and Fiber Distributed Data Interface (100 Mbps FDDI), followed by Asynchronous Transfer Mode (155 Mbps ATM) and Fibre Channel (1062 Mbps). Recently, Gigabit Ethernet (1000 Mbps) has been introduced into the industrial and consumer market. With each successive increase in speed, the physical layer of the infrastructure is placed under more stress and more limitations. In fact, the cabling installed in many environments today cannot support the demands of Fast Ethernet let alone ATM, Fibre Channel or Gigabit Ethernet.
Fiber optics provides a viable alternative to the foregoing copper based solutions. Unlike its metallic counterpart (e.g., coaxial and twisted pair topologies), fiber optics does not have the astringent speed and distance limitations. For example, Ethernet run over coax (e.g., 10 BASE2) has a maximum distance limitation of 185 m, and Ethernet run over twisted pair (e.g., 10 BASE-T and 100 BASE-TX) has a limitation of 100 m. In addition, Ethernet running at 10 Mbps has a limitation of 4 repeaters, providing some leniency in the solutions available for distance, however, Fast Ethernet only allows for two repeaters and only 5 m of cable between them. Fiber optics can greatly extend these distances with multimode fiber providing 2000 m and single-mode fiber supporting 5 km in half duplex environments, and much more (depending on transmitter strength and receiver sensitivity) in full duplex installations.
Furthermore, when using coaxial cable or twisted pair (shielded or unshielded) cable, electrical noise can be emitted by the cable, especially as connectors and ground connections age or weaken. Because fiber optics utilizes light pulses to send the signal, it is free of radiated noise, which renders it safe to install in sensitive environment. In addition, since there are no emissions to pick up and decode, it is not feasible to “tap” into it for the purposes of “eavesdropping,” and thus optical fiber can provide security protection, which makes it a good candidate for secure network installations. Another problem that is common when using copper cabling is electrical noise from other products contaminating the desired electrical signal. This can be a problem in noisy environments such manufacturing environments, and in industrial and aerospace applications. In contrast, optical fiber provides a signal that is virtually unaffected by external noise.
A typical fiber optic cable comprises a core, a cladding, a coating, a strengthener, and a protective jacket. In general, the core is the center of the cable and is the medium of propagation for an optical signal. Cores can be made of glass (e.g., silica) and/or plastic, configured as hollow or solid, and with a high refractive index. Glass based cores provide longer distances and greater bandwidth, whereas plastic provides a more affordable cable that is easier to install and splice. Typical core sizes range from 8 microns for a single mode silica glass core up to 1000 microns for a multi mode POF. The cladding generally is a material of lower index of refraction and surrounds the core. This difference in index of refraction forms a mirror at the boundary of the core and cladding. Because of the lower index, it reflects the light back into the center of the core, forming an optical waveguide. It is this interaction of core and cladding that is the heart of optical fiber transmission. For example, for the core/cladding boundary to work as a mirror, the light needs to strike it at a small/shallow angle referred to as the angle of incidence, which typically is specified as the acceptance angle (or numerical aperture, which is the sine of the acceptance angle) and is the maximum angle at which light can be accepted by the core.
The protective coating is applied around the outside of the cladding. Such coatings generally comprise a thermoplastic material for tight buffer construction and a gel material for loose buffer construction. For a tight buffer construction, the buffer is extruded directly onto the fiber, tightly surrounding it. Loose buffer construction utilizes a gel filled tube, which is larger than the fiber itself. Loose buffer construction offers a high degree of isolation from external mechanical forces such as vibration, whereas tight buffer construction provides for a smaller bend radius, smaller overall diameter, and crush resistance. To further protect the fiber from stretching and to protect it from expansion and contraction due to temperature changes, strength members can be added to the cable construction. These members typically are made from various materials from steel to Kevlar. The jacket can be applied over the strength member to protect against the environment in which the cable is installed.
As fiber deployment increases, the economy of scale for the manufacturers is driving costs down. In addition, research and development efforts continue to further reduce costs. For example, POFs provides a cost-reducing alternative to glass. In another example, optical fiber can be employed with legacy equipment and infrastructures by utilizing copper-to-fiber media converters. Media converters are devices, typically small enough in size to fit in the palm of your hand and they convert input signals from one media type and to another media type. Thus, equipment with an AUI port can utilize optical fiber transceivers. For those instances when collision domain restrictions preclude the use of fiber, a 2-port bridging device (such as Transition Networks Pocket Switch) with 10/100-BASE-T(X) on one port and fiber on the other can be utilized.
As noted above, fiber optics technology has advanced to the stage to render it a viable alternative to copper solutions. However, fiber optics, as well as its copper counterpart, lag product and consumer demand. For example, many communications systems could be expanded in performance if a device were available that would provide wide bandwidth signal delay over a long adjusted duration. A high time-bandwidth product delay line can provide processing capabilities on narrowband signals in wide spectra. Current optical technology includes fixed optical delay lines formed by fibers with no adjustment in time delay, fibers that are physically stretched over a very small percentage of total delay and switched binary combinations with discrete (e.g., course) delay steps such as delays of equal to L+L/2+L/4+L/8+ . . . +L/N, where L is the fiber length and N is an integer multiple of two. Switched binary combinations can provide more than one delay; however, discrete delay steps render the fiber susceptible to photons loss when a switch event occurs. Thus, switching delays can be a source of unreliability, and fiber length cannot be referenced to a stable wavelength.